Whopity's quote suggests that a PIC is acting illegally by not logging a flight.
Is that true?
Yes, although he didn't include the following paragraph, which states:
(2) Detailed information about each flight during which the holder of the log book acted either as a member of the flight crew of an aircraft or for the purpose of qualifying for the grant or renewal of a licence under this Order must be recorded in the log book as soon as reasonably practicable after the end of each flight.
A few people have been successfully prosecuted for breach of that particular article, although mainly, it must be said, in association with other fraudulent actions.